Electrical sound transmitter



Sept. 13, 1938. H. c. TITTLE ELECTRICAL 4SOUND TRANSMITTER Filed Nov. 16. 1934v ELI/@M079 l gui/aai 'me Patented Sept. 13, 1938 PATENT OFFICE ELECTRICAL SOUND TRANSMITTER Hulburt G. Tittle, Park Ridge, Ill., assignor, by' mesne assignments, to Aurex Corporation, Chicago, Ill., a corporation of Illinois Application November 16, 1934, serial No. 753,315

5 Claims.

'I'his invention relates to electrical sound transmitters of the kind in which a microphone is employed to pick up the sound waves and transmit the latter through various instrumentalities 5 to a receiving instrument, such as those employed in radio broadcasting, and more particularly to devices of this kind that are employed by deaf persons in order to enable them to hear.

Generally stated, the object of the invention is to provide a novel and improved construction and arrangement of instrumentalities and circuits forelectrically picking up and transmitting the sound waves in a more satisfactory manner than heretofore, by frequency modulation, and more particularly in a way tending to improve the character of the reception in the receiving instrument, by eliminating more or less of the disturbances of certain kinds that have heretofore characterized devices of this kind, and especially those that have been employed for the purpose of enabling deaf people to hear, such as those employing an ear phone as a receiver.

It is also an object to provide certain details and .features of construction and novel combinations tending to increase the general eiciency and the desirability of an electrical sound transmitter of this particular character.

To the foregoing and other useful ends, the invention consists in matters hereinafter set forth and claimed and shown in the accompanying drawing, in whichl The single figure shown is a diagram of an electrical sound transmitting apparatus embodying the principles of the invention.

As thus illustrated, the invention comprises a microphone I, preferably of the well known condenser type, having its terminals connected to the opposite ends of the coil 2, which latter is of the well known inductive air core type. A re- 40- sistor 3, having a fixed capacity 4 in shunt thereof, has one terminal connected to the lower end of the coil 2, as shown, and has its other end r terminal connected to an adjustable capacity or condenser with the lower end of the coil 6, which latter is similar to the coil 2 previously mentioned. An adjustable condenser I is connected in shunt of the coil 6, and the upper terminal ofthe coil 2 is connected at 8 with the housing or shield 9 in which the entire appara- 50 tus with its circuits is enclosed.

The triode tube I0 has a plate element II connected to the circuit connection I2, as shown, at one side of the condenser 5, and the grid element I3 is connected to the circuit I2 at the other side of the condenser 5, in the manner shown. Thev main -circuit I4 is connected to theI other well known elements I5 and I6 of the tube I0, as shown. In addition, there is a connection I'I between the casing or shield 9 and the lower side of the circuit I4, in the manner shown. Also, there is a connection I 8 between the upper terminal of the coil 6 and the lower side of the, circuit I4, with a fixed capacity .I9 therein, and there is a connection 20 parallel with the connection I8 to the primary 2| of the well known transformer employed in circuits of this kind.

The amplifying tubes 22 and 23 are of the well known type, in transmitting circuits of this kind, as are the tube 24 and the power plug 25, shown in the diagram, said plug, as indicated,

` being adapted for connection toa source of either alternating or direct current. At the receiving end of the circuit arrangement thus shown and described, as at 26, any suitable known or approved receiving instrument, such as an ordinary ear phone or a loud speaker, may be connected in the usual manner.

With the foregoing circuit arrangement, the tube I0 has three functions: first, it is a radio frequency oscillator; second, it is a demodulator 'for the frequency modulated signal; third, it serves as an audio frequency amplifier.

Preferably, instead of modulating the amplitude of the transmitting current, the frequency is modulated, to produce the signal. This is because the condenser microphone shown and described varies the frequency of the radio oscillator at audio rate, and with this method the transmission is more satisfactory than heretofore, it is found, as the arrangement and action of the instrumentalities thus employed tend to quiet the transmission, by eliminating noises or disturbances that were heretofore characteristic of electrical sound transmitters of this character, and more especially those employed for enabling deaf people to hear.

From the tube I0 to the right thereof, the entire diagram and hook-up are conventional and well known anddo not require further explanation.

Obviously, the oscillator of the system shown and described is frequency-modulated. Instead of being amplitude-modulated, as in the ordinary system, the carrier is varied in frequency (but not in amplitude) in order "to insure the desired results. As is well known, frequency modulation cannot be detected (i.. e., the audio component cannot be separated from the carrier and other components) by the usual methods. Therefore, with the invention shown and described, the system is operative to feed the signal into a tuned circuit which is tuned a little .oi the resonance point (with respect to the unmodulated carrier), and it doesnt matter whether the tuned circuit is tuned just above or just below the resonance point. Therefore, a tuned circuit, tuned off the signal frequency, acts as a detector to frequencymodulated signals.. Obviously, the steeper the resonance curve of the tuned circuit, the more sensitive will be the detection. This means that the tuned circuit and the circuits coupled (or connected) to it must have low losses. Furthermore, the bias on the amplifier tube to which the tuned circuit is connected must be high enough to take care of the strongest signal which may be impressed on the grid. The maximum bias is, of course, determined also by the available plate voltage on that tube. As far as the oscillator itself is concerned (the tube to which the condenser microphone is connected), means are provided to vary the feed-back, and therefore the strength of oscillation. Otherwise the amplitude of oscillation will be too great for the following tube to handle. vIn practice, the housing or casing 9, containing everything except the microphone l and the plug 25 and its cord, and thel receiver hooked up at 26, will rest on a desk or table convenient to a Wall or floor socket connected to a source of either alternating or direct current. The microphone I may be a separate instrument, outside of said casing, or it may be built into the casing, depending upon circumstances and requirements.

Thus the condenser microphone l is used tol tune the input circuit, at the receiving end of the radio amplifying apparatus, so that it has this function as wellas the other function of an ordinary microphone transmitter. Obviously, the condenser 5 could, by careful adjustment of the coils 2 and 6 and their associated elements, be

omitted, but for some purposes it is preferable to employ this condenser; but, in any event, the conv denser microphone I not only tunes the input circuit, but also provides the means for causing a frequency modulation, in the circuit arrangement shown and described, so that it has a double function.

Therefore, the entire invention, as shown and described, including the adjustable plate capacity 5 and 1, is built around the principle of frequency modulation, so that the latter is fundamentally essential. 'I'he invention as shown has capacity feed-back, it will be seen, which is variable, and which can be set, and which is the underlying reason for effective and stable frequency modulation, together with a variable feed-back which can be set and which tends to insure effective and stable frequency modulation.

An important aspect of this invention, as shown and described, is that there is no necessity of using a polarizing voltage, and that hence greater sensitivity is insured, inasmuch as the use of a definite frequency modulation entirely eliminates the necessity of any polarizing voltage for the microphone.

What I claim as my invention is:

1. An electrical sound transmitter, comprising a coil, a condenser microphone having its terminals connected to the opposite terminals of the coil, with one coil terminal connected 'to an input element of a tube, a separate coil and one terminall thereof connected to an output element of a tube, the other terminal of the last mentioned coil being connected to an amplification systelmfor the output, and a tuning condenser in shunt of said output coil, for producing frequency modulation as the result of sound Waves picked up by said microphone, and operative also for demodulating the signal, together with a variable condenser feed-back which can be setto insure effective and stable frequency modulation.

-2. A structure as specified in claim 1, having capacity and resistance in parallel in the input circuit between said first mentioned coil and said tube.

3. A structure as specified' in claim 1, there being a connection to ground through a condenser for said other terminal of said separate coil.

4. A structure as specified in claim 1, the other terminal of said first mentioned coil being connected to ground.

5. A structure as specified in claim 1, said amplification system having an input transformer primary to which said other'terminal of said separate coil is connected.

HULBURT C. Tl'ITLE.` 

